Method of rolling magnesium-base alloy



Patented Apr. 12, 1966 3,245,843 METHOD OF ROLLING MAGNESIUM-BASE ALLOY Guy F. Brackett, Jr., Florissant, Mo., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Filed Feb. 28, 1964, Ser. No. 348,261 6 Claims. (Cl. 14811.5)

This application is a continuation-in-part of my application Ser. No. 840,511, filed September 17, 1959, now abandoned.

The invention relates to an improved method of rolling magnesium-base alloy. It more particularly concerns an improved method of obtaining superior mechanical properties in a rolled product consisting of precipitation hardenable magnesium-base alloy.

For the purposes of the specification and claims, a magnesium-base alloy is defined as an alloy comprising at least 75 weight percent of magnesium.

The alloys employed in the present invention are those magnesium-base alloys having the precipitation hardening and strain hardening characteristics of magnesiumbase alloy comprising at least 0.4 percent by weight of thorium.

Because of their easy machinability, lightness, and favorable weight-strength ratios even at elevated temperatures, magnesium-base alloys, especially the thorium-containing magnesium-base alloys, in the form of sheet, strip and plate have found ever increasing use not only as structural panels and sheet but also as structural plate where formerly heavier metals were used. This is especially true in the case of structural plate from which high speed aircraft and missile engine mounts and aileron sections are fabricated.

Heretofore, in preparing a precipitation hardenable magnesium-base alloy, such as a magnesium-base alloy containing at least 0.4 percent by weight of thorium, in plate form, that is, in rolled form having a thickness greater than 0.249 inch, the alloy has been hot rolled from a 950 F. oven heated ingot directly to finish gage in a series of reductions. is then rapidly cooled to less than 600 F. and thermal flattened at about 625 F. for 2 hours. So-formed plate, however, tends to exhibit relatively low mechanical strength.

Aging, as used herein, refers to the heat treating of a precipitation hardenable alloy in the temperature range in which precipitation of a phase in the matrix metal occurs.

Thorium-containing magnesium-base alloy in sheet and strip form has conventionally been prepared by hot rolling the alloy by repeated passes though a rolling mill at an elevated temperature, with a heating or process anneal step between each said pass, until the rolled metal has been reduced to a thickness about 50 percent above final gage. The so-reduced metal has then been cooled to room temperature and cold rolled to final gage in successive passes and then annealed, the amount of reduction during cold rolling being only 1 to 2 percent per pass. Some disadvantages of this manner of rolling sheet and strip, when applied to large ingots, are the necessity of cutting the rolled metal into lengths suitable for handling and the repeated handling of the cut pieces that is necessary because of the number of passes required to obtain the desired total reduction of each piece. For example, in rolling sheet from a thickness of 0.3 inch to 0.064 inch, according to the conventional manner, 60 to 65 hand-fed passes of the sheet through the mill have been employed.

It is accordingly an object of the invention to provide an improved method of rolling and processing precipita- The so-obtained rolled product tion hardenable magnesium-base alloy of the type defined.

Another object of the invention is to provide an improved method of producing superior mechanical and physical properties in a rolled product of precipitation hardenable magnesium-base alloy of the type defined.

A further object of the invention is to provide a meth 0d of rolling and processing precipitation hardenable magnesium-base alloy of the type defined which is adaptable to the production of sheet and strip as well as plate.

Still a further object of the invention is to provide a method of rolling precipitation hardenable magnesiumbase alloy of the type defined which does not require multiple hand-fed passes of the metal through the rolls of a mill.

These and other objects and advantages of the invention will be apparent upon becoming familar With the following description and the appended claims.

The invention is predicated upon the discovery that by employing as an essential sequence of steps; heating a precipitation hardenable magnesium-base alloy, such as magnesium-base alloy containing at least 0.4 percent by weight of thorium, in rolled or ingot form at its solution heat treating temperature until the so-heated metal is substantially in the heat treated condition; quenching the so-heated metal to a lower temperature at which the metal hardens on working; Without further heating the soquenched metal, rolling it at the said lower temperature to effect a 20 to 50 percent reduction in thickness, where by work hardening is effected; and thereafter aging the rolled metal for a time and at a temperature at which precipitation hardening of the worked metal occurs, a rolled product is obtained which exhibits excellent mechanical and physical properties.

In producing rolled plate according to the invention, an ingot of a magnesium-base alloy which can be made precipitation hardenable during processing is subjected to a solution heat treatment as by soaking in a furnace at a temperature at which substantially complete solution of the solid solution forming alloying metal is obtained. A soaking temperature of about 950 F. generally suffices. Temperatures of about 900 F. to about 1050 F. may be used. Soaking time varies with the volume and the shortest dimension of the ingot and generally ranges from about 10 to 20 hours for a 2000 lb. ingot including the time taken to heat the metal from room temperature to the solution heat treating temperature.

The solution heat treated ingot While still hot (about 800 to 950 F.) is hot rolled by passing the metal between conventional rolls while the metal is at a temperature at which it can recrystallize using as many passes as necessary to obtain the desired thickness reduction. Each hot pass produces a reduction in thickness of an amount less than that which would crack or fracture the metal as understood in the art of hot rolling a magnesium-base alloy. The number of hot passes will vary according to the initial thickness of the ingot and the ultimate thickness desired.

The hot rolling is discontinued when the thickness to which the metal has been brought is in the range of from about to 200 percent of the final or ultimate thickness, more preferably in the range of 125 to 143 percent of the final thickness. If the ingot is adequately heat treated before rolling and if the ingot is then rolled to the penultimate thickness in large passes without delay, it is generally found that little or no work hardening is imparted by the hot rolling and the so-rolled metal is still in the fully heat treated and soft temper condition. If the metal should become cooled during the hot rolling sequence to the point that Work hardening occurs, it is essential according to the process of the invention that the rolled metal be heat treated for the requisite time to return the metal to the heat treated 3 and soft temper condition. In a practical manner, such additional heat treating is carried out when necessary, after rolling the metal to the penultimate thickness. As the rolled metal will now be much thinner than the ingot and already at an elevated temperature, a short heat treating time suflices, for example, about one hour.

The metal is then cooled rapidly as by an air or water quench from the solution heat treating temperature to a lower temperature at which work hardening occurs. The lower temperature is advantageously in the range of 600 to about 725 'F., an even more preferred range being 650 to 700 F. Temperatures above about 550 F. and preferably above 600 F., are generally employed to avoid cracking the metal when it is rolled commercially.

The quenched metal is then rolled to its ultimate thickness, 2. total reduction of 20 to 50 percent being carried out using one or more passes through conventional rolls. Carrying out the reduction in the least possible number of passages is generally preferred to obtain desirable properties in the metal as well as to minimize handling steps.

Thereafter, the so-rolled plate is thermally flattened under temperature and time conditions suitable to aging the alloy in a beneficial manner and at the same time at a sufliciently elevated temperature that flattening of the plate can take place under moderate to small stress loads. Such a compromise calls for a temperature in the range of 400 to about 650 F.

In the event the plate is of heavier gage than about 0.75 to 1 inch thickness and formed of an alloy having a maximum aging temperature more than about 50 F ahrenheit degrees below the lower rolling temperature employed in the final reduction step, it is much to be preferred that the plate be quenched to the aging temperature immediately after rolling.

The concurrent thermal flattening is carried out, for example, by stacking rolled plate on a flat steel plate or platform conveniently loaded on a cart or truck, placing a flat weighted steel plate on top of the stack and moving the whole assembly into an oven wherein the assembly is brought to and maintained at a temperature which is suitable both for aging and for flattening under the pressure of the weighted steel plate in about one to two hours with a minimum of annealing effect.

Some thorium-containing magnesium-base alloys which can be satisfactorily rolled and processed according to the present invention are listed in Table I by their ASTM designations. Also listed are some suitable temperatures for concurrent aging and thermal flattening.

TABLE I Mg alloy: Aging-flattening temp, F. HM21 600 HK31 400 ZH11 500 HKll 500 since tremendously long run-out tables are not practical,

nor is the handling of sheet cut in many pieces economical.

The above described method of the invention is adapted to the production from ingot of long lengths of sheet or strip in coiled form as follows:

An ingot of a precipitation hardenable magnesium-base alloy is subjected to a solution heat treatment in the same manner as that described above for the preparation of plate.

The ingot is then hot rolled in a conventional manner,

large reductions per pass being employed to minimize handling of the metal.

When the rolled metal sheet or strip has elongated on rolling to the extent that coiling becomes necessary, the metal is generally thinner than about 0.4 inch. If the metal 'at the same time has been brought to the penultimate thickness desired, which is about to 200 percent of the ultimate thickness, the rolled metal is coiled on a mandrel as it leaves the rolls of the mill on the last hot pass. More preferably, the penultimate thickness is about 143 to 167 percent of the final thickness. The coiled metal is then given a solution heat treatment in a 950 F. oven to bring it to the heat treated condition. Since the metal is already at an elevated temperature, a heating time of one hour is generally suflicient.

On removing the coil from the oven the heat treated metal is immediately cooled quickly to a lower temperature at which the metal is hardened by working, e.g., a temperature in the range of 600 to 725 F. Cooling is conveniently accomplished by applying a water quench to the metal as it is uncoiled and fed into the rolls of a mill.

The quenched metal is then reduced in thickness by about '20 to 50 percent in one pass at the said lower temperature, and the metal coiled, or, cut into suitable flat pieces, as desired.

The so-rolled and coiled metal is then aged in an oven at a temperature suitable for precipitation hardening the metal in a reasonable time, e.g., one to four hours. A temperature in the range of 300 to 600 F. will usually sufiice. If the rolled metal has been uncoiled and cut into flat pieces, they are stacked and similarly aged.

If, on the other hand, it is desirable to quench the metal to the aging temperature as it exits from the mill after rolling it at the lower rolling temperature, this is conveniently accomplished by air or water quenching the metal as it leaves the rolls on the final pass and prior to coiling and aging.

As a further modification of the process of producing sheet or strip of precipitation hardenable magnesium-base alloy of the type defined, especially thinner gage metal, the hot rolled metal on reaching a length requiring coiling but still requiring further hot rolling is coiled and reheated then uncoiled before each hot pass until the penultimate thickness is reached. The remainder of the rolling process is then carried out as described above for sheet and strip.

Examples The following specific example is illustrative of the production of rolled plate according to the practice of the invention:

A scalped cast ingot of magnesium-base alloy 12 inches thick, 40 inches wide and 70 inches long weighing 2,250 pounds and having the nominal composition 2 percent of thorium, 0.6 percent of manganese, the balance magnesium was solution heat treated at 900 to 950 F. for about 16 hours. The so-heated ingot while at a temperature of about 925 F. was subjected to hot rolling on a breakdown mill equipped with 27 inch diameter work rolls. A roll lubricant of a 10 percent water solution of a liquid polyglycol was applied to the rolls which were maintained at a temperature of about F. The roll surface speed was about 600 feet per minute. In the first few passes the ingot was passed between the rolls sideways which widened the ingot to 51 inches and at the same time reduced its thickness to 9.4 inches. The partially reduced ingot was then turned 90 degrees in the plane of the axes of the rolls and sent between the rolls in the endways or lengthwise direction in 14 passes which reduced the ingot to 1.25 inch plate. During these passes the temperature of the rolled metal fell to about 880 F. The so-rolled plate was then quenched by means of water sprays to about 675 F. and given three light reductions from 1.25 inch to 1.15 inch, to 1.10 inch and to 1.0 inch. The temperature of the Comparison finished gage 1 inch plate was about 530 F. upon exiting from the mill on the final pass. The so-rolled plate was then thermally flattened by placing it on a flat machined steel plate mounted on a mobile platform, covering the rolled plate with a flat weighted steel plate, and moving the whole assembly into an oven in which the rolled plate was brought to a temperature of 600 F. for about an hour. The mechanical properties of the so-rolled and flattened plate as determined at various temperatures are listed in the table after Example 1. By way of comparison, a second ingot of the same alloy was formed into 1 inch plate according to the prior art wherein the ingot is hot rolled to final gage (1 inch), quenched to 600 F. and thermally flattened for 2 hours at 625 F. Physical properties of this plate were determined and the values are listed in the table after comparison.

In additional experiments, flat rolled plates of various thicknesses were prepared from ingots of a magnesiumbase alloy have the ASTM designation HM21, according to the practice of the invention. In each experiment the hot rolling was carried out in the hot working range to bring the plate to about 25 percent above the final gage. The so-rolled plate was then quenched to about 675 F. The plate was reduced to final gage in three light passes and the final gage plate thermal flattened in a 650 F. oven for four hours, the metal being at 650 F. for about one hour. Room temperature properties of the so-obtained plates are listed also in Table II for these examples which are numbered 2, 3, 4 and 5.

TABLE II these passes the metal widened to 53 inches, lengthened to about 160 feet and cooled to slightly above 800 F. On leaving the rolls of the mill on the twenty-second pass, the metal, while still above 800 F., was coiled on a mandrel having a diameter of 24 inches. The coiled metal was placed in an oven and annealed at 800 for one hour. The annealed coiled metal was then held in a 650 F. oven to await further rolling. The coiled metal was then uncoiled and reduced in thickness 20 to 25 percent in a single pass through the mill, the metal entering the rolls at about 650 F. The so-reduced metal was coiled as it emerged from the mill, while still hot. The coiled metal was again annealed in an 800 F. oven, held in a 650 F. oven to await further rolling and the preceding rundown rolling steps repeated through the penultimate pass of the rolled metal through the mill which brought the rolled metal to a thickness of 0.062 inch or 154 percent above the predetermined ultimate gage. After the said penultimate pass the metal was coiled hot as before and placed in an oven and heated to 900 F. for one hour. Then the coiled metal was removed from the oven, transferred to the rolling mill and quenched by a water spray from about 800 to 850- F. to a lower temperature above about 725 F. as it was uncoiled and fed into the rolls of the finishing mill on the final pass. In this last pass the thickness of the metal was reduced percent to 0.040 inch while the temperature of the metal was reduced to about 650 F. by passing it through cooled rolls slowly enough to bring about the desired cooling. The so-rolled metal was fur- Physical Properties (Strength values in 1000s of p.s.i.)

Dlrection of Test 1 Example N0. 70 F.

Percent TYS TS CYS NNMBQMMMM N1 i pwwww ws s s w l Direction of test in determining physical properties of sample in longitudinal (L) direction of rolling or the transverse (T) I direction.

2,300 pounds and having the nominal composition of 3 percent by weight of thorium, 0.6 percent by weight of zirconium, the balance magnesium was solution heat treated in a 950 F. oven for about 16 hours. The so-heated ingot while at a temperature in the range of 900 to 950 F. was then subjected to hot rolling on a breakdown mill equipped with 27 inch diameter work rolls. A roll lubricant consisting of a 10 percent water solution of polyglycol was applied to the rolls which were maintained at a temperature of about 140 F. The roll surface speed was about 600 feet per minute. In the first few passes the ingot was passed between the rolls sideways which widened the ingot to about 51 inches and at the same time reduced its thickness to about 9.5 inches. The so-rolled ingot was then turned 90 degrees in the plane of the axes of the rolls and sent between the rolls in the endwise direction in 22 passes which reduced the thickness of the metal to 0.300 inch. During ther water-quenched as it emerged from the mill to a temperature below 400 F., dried by a stream of air, and coiled on a 24 inch diameter mandrel. The quenched coiled metal was then aged at 400 F. for four hours. The physical properties of the so-produced sheet are listed in Table III as test No. 6 along with, as a comparison, the properties of sheet of the same thickness and alloy composition prepared according to the previously known method as follows: A scalped cast ingot similar to that described above was heat treated and rolled in a similar manner on a breakdown mill but without coiling until the metal was brought to a thickness of about 0.4 inch. The so-rolled metal was then cut into 10 foot lengths and allowed to cool to room temperature. The so-prepared pieces of sheet were then pickled, wire brushed and annealed at 800 F. and hand fed through a rundown mill while the metal was still at a temperature above about 650 F. The rolled metal was immediately sent through the mill 2 to 4 more addi- TABLE III Example No. Percent TYS TS CYS Band 6 4. 5 33. 9 40. 9 30.0 5. 5 Comparison 5. 30. 6 38. 21. 5

Percent E percent elongation in 2 inches. 1 'lllYs tensile yield strength in 1,000s of pounds per square TS=ultimate tensile strength in 1,000s of pounds per square inch.

CYSzcompression yield strength in 1,000s of pounds per square inch.

Bend '1:

minimum radius of 90 degree bend made at room temperature without fracture thickness of specimen I claim: 1. As the method of producing an improved rolled product of magnesium-base alloy comprising at least 0.4 percent by weight of thorium and having precipitation hardening and strain hardening characteristics, the sequence of steps which comprises:

bringing the metal to its solution heat treating temperature in the range of 900 to 1050 F.;

quenching the so-heated metal to a lower temperature at which the metal hardens on being worked, said lower temperature being in the range of 550 to 725 F.;

without further heating the quenched metal, reducing the thickness thereof by about to 50 percent by passing the metal through the rolls of a mill while the metal is at said lower temperature thereby to work harden the metal while bringing the metal to a predetermined ultimate thickness; 7

and aging the so-worked metal for from 1 to 4 hours at a temperature in the range of 400 to 650 -F. at which precipitation hardening of the metal occurs. 2. As the method of producing an improved rolled product of magnesium-base alloy comprising at least 0.4 percent by weight of thorium and having precipitation hardening and strain hardening characteristics, the sequence of steps which comprises:

bringing the metal to its solution heat treating temperature in the range of 900 to 1050 F.;

quenching the so-heated metal to a lower temperature at which the metal hardens on being worked, said lloglerFtemperature being in the range of 550 to without further heating the quenched metal, reducing the thickness thereof by about 20 to 50 percent by passing the metal through the rolls of a mill while the metal is at said lower temperature thereby to work harden the metal while bringing the metal to a predetermined ultimate thickness; quenching the so-rolled metal from the work hardening temperature to the precipitation hardening temperature; i

and aging the so-worked metal for from 1 to 4 hours at a temperature in the range of 400 to 650 F. at which precipitation hardening of the metal occurs.

3. As the method of producing an improved rolled product of magnesium-base alloy comprising at least 0.4 percent by weight of thorium and having precipitation hardening and strain hardening characteristics, the sequence of steps which comprises:

hot rolling the metal, by means of multiple passes through the rolls of a mill, into rolled form having from about to 200 percent of the ultimate rolled thickness; bringing the rolled metal to its solution heat treating temperature in the range of 900 to 1050 F. for a time sufficient to bring the metal to the heat treated condition; quenching the so-heated metal to a lower temperature at which the metal hardens on being worked, said lower temperature being in the range of 550 to 725 F.; without further heating the quenched metal, rolling. the

quenched metal to the ultimate thickness thereby reducing the thickness of the metal 20 to 50 percent while the metal is at said lower temperature thereby to work harden the metal; and aging the so-wor-ked metal for from 1 to 4 hours at a temperature in the range of 400 to 650 F. at which precipitation hardening of the metal occurs. 4. As the method of producing an improved rolled product of magnesium-base alloy comprising at least 0.4 percent by weight of thorium and having precipitation hardening and strain hardening characteristics, the sequence of steps which comprises:

bringing the metal to its solution heat treating temperature in the range of 900 to 1050 F. for a time sufiicient to bring the metal to the heat treated condition; hot rolling the metal by means of multiple passes through the rolls of a mill into rolled form having from about 125 to 200 percent of the ultimate rolled thickness while maintaining the metal in the heat treated condition at a temperature above 800 F.; while the so-rolled metal is in the heat treated condition, quenching the metal to a lower temperature at which the metal hardens on being worked, said lower temperature being in the range of 550 to 725 F.; without further heating the quenched metal, rolling the quenched metal to the ultimate thickness reducing the thickness of the metal 20 to 50 percent while the metal is at said lower temperature thereby to work harden the metal; and aging the so-worked metal for from 1 to 4 hours at a temperature in the range of 400 to 650 F. at which precipitation hardening of the metal occurs. 5. As the method of producing an improved rolled product having a thickness less than about 0.249 inch and said product consisting of magnesium-base alloy comprising at least 0.4 percent by weight of thorium, and having precipitation hardening and strain hardening characteristics, the sequence of steps which comprises:

hot rolling the metal by means of multiple passes through the rolls of a mill to a penultimate thickness of about 143 to 167 percent of the ultimate rolled thickness; coiling the hot rolled metal on a mandrel; bringing the coiled metal to its solution heat treating temperature in the range of 900 to 1050" F. for a time sufficient to bring the metal to the heat treated condition; quenching the so-heated metal from its solution heat treating temperature to a lower temperature at which the metal hardens on being worked, said lower temperature being in the range of 550 to 725 F., said quenching being carried out as the metal is uncoiled and fed into the rolls of a mill; rolling the quenched metal to the ultimate thickness thereby reducing the thickness of the metal from 30 to 40 percent while the metal is at said lower temperature thereby to work harden the metal; coiling the so-worked metal; and aging the coiled metal at a temperature in the range of 400 to 650 F. for about an hour. 6, AS the method of producing an improved rolled 9 10 plate of magnesium-base alloy comprising at least 0.4 perwithout further heating the quenched metal, rolling the cent by weight of thorium and having precipitation hardquenched metal to the ultimate thickness thereby ening and strain hardening characteristics, the sequence of reducing the thickness of the metal from 20 to 30 steps which comprises: percent while the metal is at said lower temperature hot rolling the metal by means of multiple passes 5 thereby to work harden the metal;

through the rolls of a mill to a penultimate thickness and aging the rolled metal at a temperature in the of about 125 to 143 percent of the ultimate rolled range of 400 to 650 F. for about an hour. thickness; bringing the rolled metal to its solution heat treating References Cited y the Examine! temperature in the range of 900 t0 1050 F. fOI a 10 UNITED STATES PATENTS ill'llzfiililofllnfllent to bring the metal to the heat treated 2,934,461 4/1960 Burke at al- 148 115 2,934,462 4/ 1960 Barbian et a1 148-115 quenching the so-heated metal from its solution heat treating temperature to a lower temperature at which the metal hardens on being worked, said lower tem- 15 BENJAMIN HENKIN Pnmary Exammer' perature being in the range of 550 to 725 F.; DAVID L. RECK, Examiner. 

1. AS THE METHOD OF PRODUCING AN IMPROVED ROLLED PRODUCT OF MAGNESIUM-BASE ALLOY COMPRISING AT LEAST 0.4 PERCENT BY WEIGHT OF THORIUM AND HAVING PRECIPITATION HARDENING AND STRAIN HEARDENING CHARACTERISTICS, THE SEQUENCE OF STEPS WHICH COMPRISES: BRING THE METAL TO ITS SOLUTION HEAT TREATING TEMPERATURE IN THE RANGE OF 900 TO 1050*F.; QUENCHING THE SO-HEATED METAL TO A LOWER TEMPERATURE AT WHICH THE METAL HARDENS ON BEING WORKED, SAID LOWER TEMPERATURE BEING IN THE RANGE OF 500 TO 725* F.; WITHOUT FURTHER HEATING THE QUENCHED METAL, REDUCING THE THICKNESS THEREOF BY ABOUT 20 TO 50 PERCENT BY PASSING THE METAL THROUGH THE ROLLS OF A MILL WHILE THE METAL IS AT SAID LOWER TEMPERATURE THEREBY TO WORK HARDEN THE METAL WHILE BRINGING THE METAL TO A PREDETERMINED ULTIMATE THICKNESS; AND AGING THE SO-WORKED METAL FOR FROM 1 TO 4 HOURS AT A TEMPERATURE IN THE RANGE OF 400 TO 650*F. AT WHICH PRECIPITATION HARDINING OF THE METAL OCCURS. 